The invention relates to a radio front-end circuit and more specifically to a reconfigurable radio front-end circuit.
Radio front-end circuits are typically designed for low-noise operation. Such operation requires the transconductance-source-resistance product, gmxc2x7RS, of the input device, e.g. a bipolar or MOS transistor, of a front-end circuit to be much larger than 1. In its turn, this leads to a high current consumption as the quiescent current determines the transconductance of the input device.
Frequently, the signal strength is much higher than the sensitivity level of the front-end circuit. In such cases, the bias of the front-end circuit could be reduced in order to lower the supply current. The noise level would increase, but the signal-to-noise ratio could nevertheless be kept high enough not to degrade the received signal.
However, changing the front-end circuit bias degrades input matching and reduces linearity and band width to such an extent that it is hardly ever used. Lowering the quiescent current of the input device will lower the transit frequency of the device. For high-frequency applications, such as cellular telephony, this is no option because the transit frequency margin is too low. The current density of the input device has to be maintained which implies that it will be necessary to switch between two different front-end circuit structures. The switch necessary for altering the front-end circuit structure will contribute to the noise level and will typically degrade the low-noise performance. Furthermore, linearity is often also an issue.
The object of the invention is to bring about a reconfigurable radio front-end circuit in which the power consumption is reduced when the signal strength is high.
This is attained by means of the front-end circuit according to the invention in that the amplifiers of the front-end circuit are switched on or off via the mixers instead of being switched on or off by switching the bias current or bias voltage of the amplifiers or by introducing separate switches or multipliers in the signal path.
The advantage of this is that the input impedance of the amplifier changes less than if its supply current is switched. Moreover, no new circuit elements are introduced in the signal path, which would otherwise destroy the noise performance.